<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
   function foo(s){
       alert(s)
   }
   foo();

   var f = foo; //是没有括号，有括号表示函数就执行，函数是一个数据类型，
   f('你好');


   var o = {
    username:'frank'
   }

   var o2 = o;
   o2.username;

</script>
</body>
</html>